When it launched in 1957, the Mercedes-Benz 300d—chassis code W189—replaced the 300c (W186) as the German automaker’s flagship luxury sedan.

Engineered and built to the highest possible standards, it was on par in both size and appointments with Rolls-Royce’s contemporary Silver Cloud.

Although favored by dignitaries and offering a variety of options to coddle rear occupants, the 300d was also a true driver’s car.

After all, it shared DNA with the stellar 300 SL “gullwing” coupe, including its rigid chassis design, suspension componentry, and excellent M186 inline six-cylinder engine.

The chassis employed Mercedes-Benz’s familiar x-shaped steel-tube frame with the wheelbase extended by four inches over that of the preceding 300c to increase rear legroom.

The sophisticated suspension was fully independent and featured a driver-adjustable, rear, load-leveling function.

A pedal-operated lubrication system greased friction points in the chassis to preserve the quiet ride.

The robust...